There is a demand in many industries to improve the sensitivity of passive sensors that provide real-time information about their environment such as those measuring stress and strain, which can be induced by movement in structures, pressure change and temperature change. The traditional method for sensing strain has relied on piezoelectric strain gauges, which generate a voltage indicative of applied strain. However, large-scale piezoelectric-based systems suffer from a number of problems, including the cost of the piezoelectric sensors, the size of the sensors, the signal attenuation over long cables, spurious signals caused by electromagnetic interference, and the expense of high-speed electric cabling to connect sensors in larger arrays. A number of the problems of the piezoelectric-based methods can be overcome by using techniques based on optical equipment. For example, sensors can be connected with optical fibre, which has a far greater bandwidth than electric cabling and is immune to electromagnetic interference. Optical systems can be constructed of inexpensive, commoditised telecommunications equipment. Strain measurements can be made in optical systems using relatively inexpensive Fibre Bragg Gratings.
Fibre Bragg Gratings are created by burning (ie writing) a periodic pattern along a segment of optical fibre using high-intensity ultraviolet light; the pattern consists of alternative lines of high and low refractive index, which is a Bragg grating. A Bragg grating is a highly colour-selective mirror: light passing through the periodic structure is either transmitted or reflected depending on its wavelength. The wavelengths that are reflected can be chosen in the design of the grating: for example, the extent to which light of one wavelength (or colour) is reflected depends on the spacing of the lines that make up the grating.
A Fibre Bragg Grating can be used as a sensor because the line spacing, and thus the amount of reflected light at one selected wavelength, changes with the optical length of the fibre, which in turn changes with mechanical strain or temperature.
Fibre Bragg Gratings have been proposed as ultra-sensitive static and dynamic strain detectors for a variety of applications, such as underwater acoustic array sensors, embedded monitoring of smart structures in civil and aerospace industries, ultrasonic hydrophones for medical sensing, submarine surveillance and seismic sensors for geophysical surveys. The benefits over the piezoelectric strain sensors include their smaller cross-sectional area, their scalability to large arrays, and their suitability for electromagnetic interference-sensitive and hazardous environmental applications. In addition, optical sensor arrays can be remotely interrogated and optically multiplexed using standard, commoditised, telecommunications equipment. Early demonstrations were based on changes in the gross Bragg wavelength, as the gratings were perturbed by strain and temperature. As interrogation techniques became more sophisticated, various signal processing and active fringe side locking schemes were employed, which dramatically improved the resolution of these sensing schemes.
United States patent application number 2001/0013934 discloses an interferometric sensing device using a broadband switched optical source and sensing interferometers which can be formed in optical fibre Bragg Gratings. A matched interferometer contains a phase modulator and the sensing interferometers have an optical path difference approximately equal to the optical path difference in the matched interferometer. An optical interference signal at a different wavelength is returned to a detector by each of the sensing interferometers. Each interference signal is representative of the difference between the optical path length of the sensing interferometer and that of the matched interferometer and this can be used as a measurement signal. This approach is limited by a number of difficulties, including: (i) achieving accurate control of the path length difference between the sensing and reference interferometer; (ii) reducing acoustic noise arising from the reference interferometer; and (iii) improving the limited strain resolution and dynamic range arising from the use of white light.
Another approach is described by G. Gagliardi et al in Optics Express, Volume 13, No. 7 where radio-frequency modulation techniques are used to interrogate Fibre Bragg Grating structures. Strain measurements are made by obtaining a measure of the changes in Bragg wavelength from laser radiation reflected by the grating. This approach is limited by a number of difficulties, including: (i) the radio-frequency modulation needs to be very high compared to the sensitive bandwidth of the Fibre Bragg Grating; and (ii) the achievable sensitivity is poor as the frequency discrimination and strain discrimination of the error signal is limited by the sensitive bandwidth of the Fibre Bragg Grating.